1. Technical Field of the Invention
The present invention relates to a display device which displays an output, such as characters or image information, from an output device such as a computer, a television (TV), a video player, an optical disc drive, a TV phone, a TV or computer game and the like, and more particularly, the present invention relates to a virtual screen (VS) display device which enlarges and displays an image in space as an observed image while a background behind the VS display device can be simultaneously observed. The VS display device can be a VS stereoscopic display device and is applicable to a head-up display (HUD), a head mount display (HMD), a projector type color image display device, a liquid crystal projector, a portable display and other display devices.
2. Description of Related Art
Conventional virtual screen display devices are known in which an image displayed on a cathode ray tube (CRT), a liquid crystal display element (LCD) or another image display is enlarged and displayed in space as an observed image, rather than a displayed image as displayed on the CRT or LCD or the like, by using a hologram combiner or another combiner and such that a background located behind or on the rear side of the combiner can be simultaneously observed or seen while viewing an image displayed on the virtual screen display device. Also, a display device for enlarging and projecting a three-dimensional image is known.
For example, a display system described in Japanese Patent Application Laid-open Publication No. 1-147421/1989 uses a volume phase hologram or the like as a light flux combining element to polarize only a light flux having a specified wavelength and project the specified wavelength light flux toward an observer. The observer can observe the displayed information and another person's face in the same field of view while the displayed information can not be viewed or read by the other person.
A display device described in Japanese Patent Application Laid-open Publication No. 8-201722/1996 is provided with an optical filter including a plurality of surface-splitting filter portions having different transmission wavelengths and a hologram combiner which displays virtual images of a plurality of display images which are transmitted through the filter portions of the optical filter in a different position or substantially the same position as that seen by an observer.
A three-dimensional image projecting device described in Japanese Patent Application Laid-open Publication No. 9-243961/1997 is provided with a projecting lens for enlarging and projecting a three-dimensional image, a first concave mirror forming an enlarged virtual image of the image output from the projecting lens and a second concave mirror forming a real image of an image output from the projecting lens. It is further described that in the three-dimensional image projecting device, first and second holograms are used instead of first and second concave mirrors such that the virtual image of the projecting lens is positioned at the center of curvature of the second concave mirror and such that the first and second holograms are arranged to be connected to each other.
In JP 1-147421/1989, since a volume phase diffraction grating is used as the combiner, a substantially plane surface must serve in the same manner as a spherical surface. Therefore, a display light flux is directed toward the observer's eyes even when the combiner is arranged vertically. However, the display has a disadvantage in that the display can be performed only within a specified wavelength.
In JP 8-201722/1996 described above, an optical filter similar to a display information color filter is divided into a plurality of regions, but the image divided into regions on the optical filter can be displayed at substantially the same position by laminating or exposing multiple layers of the hologram combiner for each wavelength. Therefore, a full-color display is possible. On the other hand, since the image displayed on an LCD or CRT is displayed as a virtual image as it is, the LCD or CRT must be enlarged while maintaining high resolution which results in significantly increased cost. Furthermore, a broad, uniform and highly luminous light source is necessary for such an apparatus for proper viewing of the virtual image. Such a light source is technically complicated and significantly increases the cost of the apparatus.
In JP 9-243961 described above, although the structure is applied to a three-dimensional image projecting device, a field optical element may include a concave mirror or a reflective hologram, i.e., an optical system of a so-called VS (virtual screen) display device. In this device, two components of the concave mirror or the hologram are used to enlarge an observation view region. Specifically, the optical system includes the first concave mirror for forming the enlarged virtual image of the projecting lens and the second concave mirror for forming the real image of the projecting lens, and the observation view region is enlarged by locating the virtual image of the projecting lens in the curvature center of the second concave mirror. However, in this device, a projected object is enlarged about 1.94 times while the observation view field is enlarged twice or about 2.0 times. Therefore, a distinguishing effect cannot be obtained and the image is not easily viewable. Strangely, the magnitude of the projecting lens is not expressly described in this reference, although the projecting lens actually has a size of an exit pupil. Therefore, it is uncertain how much or to what degree the observation view region is enlarged as compared with the related devices described above. Even assuming that an F number of the projecting lens is set to F=1.4, a focal distance f is 300 mm, then the observation view region is 429 mm when the diameter of the exit pupil is 214 mm. Although this is a good value, such level can be realized without using two concave mirrors as described later in the preferred embodiments of the present invention. Moreover, the use of two concave mirrors and locating the virtual image of the optical projecting element formed by the first concave mirror in the curvature center of the second concave mirror excessively restricts the freedom in designing an optical system layout. This means that the possibility of developing various applications is remarkably reduced.
In another related device, conventional projection type color image display devices using a liquid crystal panel are known to use either a three-plate system or a single-plate system. In the three-plate system, three liquid crystal panels are used, and three color component images, i.e., red, green and blue images of a color image to be displayed are displayed on the individual liquid crystal panels. These three liquid crystal panels are separately irradiated with red, green and blue lights, and the red, green and blue lights transmitted through the liquid crystal panels are focused by a common image forming lens to synthetically form an image on a screen, and thereby a color image is displayed.
In the single-plate system, one liquid crystal panel (single-plate liquid crystal panel) is used. Red, green and blue component images are simultaneously displayed on the single-plate liquid crystal panel. The red component image is irradiated with a red light, the green component image is irradiated with a green light and the blue component image is irradiated with a blue light. The red, green and blue light for irradiation is obtained from a single white light source and by using a color separator as described below. Light fluxes transmitted through the single-plate liquid crystal panel are focused by a common image forming lens to form an image on a screen, and thereby a color image is displayed.
In the three-plate system, since the red, green and blue component images are separately displayed on the three liquid crystal panels, each component image can be displayed with a high picture element density. The quality color image having a high picture element density can be displayed, but the cost is significantly increased because the three liquid crystal panels are used.
The single-plate system can be provided for a relatively low cost. However, since the three color component images are simultaneously displayed on one liquid crystal panel, it is difficult to increase the picture element density of the displayed color image.
Furthermore, in both the single-plate system and the three-plate system, a white light from a white light source is color-separated into red, green and blue lights. Since a space for the color separation and an amount of heat generated at the white light source are large, a cooling device for cooling the white light source exclusively and a large cooling space are necessary, which greatly enlarges the color image display device.
Moreover, in the single-plate system, color-separated light fluxes are made incident at mutually different angles on the single-plate liquid crystal panel. Therefore, the color separation of the white light is very difficult.
In addition, as described above, display devices for displaying a stereoscopic image are known. Various three-dimensional display systems are disclosed in the article titled "Three-dimensional Display--Various Systems and Application to Television," Journal of the Television Society Vol. 41, No. 7, p. 610-618 (1987). This article includes a picture of a stereoscopic vision system without glasses using a concave mirror in FIG. 2(i) on page 611. It is further described on page 612 that there is a system in which screen images of two projectors are formed in an interval between both eyes, respectively, by using a large concave mirror or a large convex lens of FIG. 2(i).
In a stereoscopic image display device described in Japanese Patent Application Laid-open Publication No. 8-5956/1996, space modulation elements (display elements) for right and left eyes are disposed substantially at a right angle relative to each other, and between which a half mirror combiner is disposed. A light emitting device as a back light is placed on the side of a rear surface of each space modulation element and each light emitting device is provided with a light emitting region for one eye and a non-light emitting region for the other eye. An optical element with a directivity for enlarging the light emitting region is disposed between the space modulation element and the light emitting device, and images presented for right and left eyes are directed toward the right and left eyes, respectively, in such a manner that a stereoscopic image can be seen without using polarizing glasses or the like. It is also described that the back light is lit in a time-sharing manner and that an optical control device is provided with the light emitting region for one eye and the non-light emitting region for the other eye is disposed between each light emitting device and the space modulation element and that the optical element having the directivity for guiding the light from each transmission region toward each eye is applied before a half mirror. A transmission region of the optical control device is time-shared and a polarizing device is provided on a front surface of a display surface of each display element to introduce orthogonal straight polarized lights into the half mirror and the optical control device having a region which is divided into right and left regions for passing only each polarized light is placed on the front surface of the half mirror.
A single-eye observation view distance-adjusting display device described in Japanese Patent Application Laid-open Publication No. 8-146348/1996 is provided with at least an original image forming unit, a projection lens and an optical pupil mapping device. The optical pupil mapping device is arranged in such a manner that a pupil of the projection lens is mapped to at least one eye pupil of an observer and that a single-eye observation view distance is adjusted independently of a position of the optical pupil mapping device by adjusting an image forming position of the projection lens for an original image. A natural stereoscopic view can be realized by matching the single-eye observation view distance to a both-eye observation view distance and a clear stereoscopic image can be obtained without using special glasses or a lenticular lens.
In Japanese Patent Application Laid-open Publication No. 8-186849/1996, a transmission type stereoscopic visual device without using polarizing glasses is disclosed. Polarizing members each having a polarizing direction which is perpendicular to an adjoining polarizing direction are arranged in a stripe configuration on the side of a rear surface of a screen and lenticular lens plates having a pitch equal to a width of the stripe are disposed on the side of a front surface of the screen. A left-eye projection light for a left eye and a right-eye projection light for a right eye each having a polarized light aligned in the polarizing direction are projected on the screen via a projector.
In JP 8-5956 described above, the space-modulation elements (display elements) for both eyes are arranged at a substantially right angle between which the half mirror combiner is disposed. The light emitting device for the back light is placed on the side of the rear surface of each space modulation element. Although it is not clear why, each light emitting device is provided with the light emitting region for one eye and the non-light emitting region for the other eye. The optical element with the directivity for enlarging the light emitting region is disposed between the space modulation element and the light emitting device. Images presented for right and left eyes are directed to the right and left eyes, respectively. Therefore, a stereoscopic image can be seen without using the polarizing glasses or the like, but the luminous energy is reduced by half or more because the half mirror is used. Furthermore, a control of the light emitting region and the non-light emitting region in order not to overlap right and left images and other various features are described. However, this has little effect although they have a relatively large-scale constitution.
JP 8-146348 describes, by referring to the Journal of Television Society article described above, that it is known that the screen images of two projectors are formed in the interval between both eyes by using a concave mirror or a positive lens. Then, it is described that the device is constituted in such a manner that when the position in which the image of an original image is formed by the projection lens is adjusted, the single-eye observation view distance is adjusted independently of the position of the optical pupil mapping device. It is further described that by matching the single-eye observation view distance with the both-eye observation view distance, a natural stereoscopic view is realized and a clear stereoscopic image can be obtained without using special glasses or the lenticular lens. However, although the matching of the single-eye observation view distance with the both-eye observation view distance and its method are detailed, it is not described in detail how two view regions are arranged.
In JP 8-186849, the polarizing members each having the polarizing direction which is perpendicular to the adjoining polarizing direction are arranged in the stripe configuration on the side of the rear surface of the screen while the lenticular lens plates having the pitch equal to the width of the stripe are disposed on the side of the front surface of the screen. The left-eye projection light for the left eye and the right-eye projection light for the right eye each having a polarized light aligned in the polarizing direction are projected on the screen with the projector. Therefore, the transmission type stereoscopic visual device without using the polarizing glasses is realized. However, since the polarized light is used and the screen is divided into two sections, the luminous energy is reduced to a quarter or even more. Furthermore, since the device is of the rear-surface projection type, the total system is disadvantageously enlarged (lengthened).